This invention relates to radioactive well logging methods and apparatus for investigating subsurface earth formations traversed by a borehole. The invention more particularly relates to compensation of drift in a scintillation type gamma ray detector and compensation for salinity effects in the borehole.
It is well known that in oil and gas wells physical characteristics of the formation surrounding the well and the chemical content of the formations and fluids in the formations can be determined from radiation emanating from the formation. The radiation detected may be either radiation naturally originating in the formation or may be induced radiation caused by irradiating the formation during the well logging operation.
Scintillation detectors have long been used for analyzing the energy content characteristics of the radiation from the formation to enable the determination of a spectrum of pulse heights being emitted by a formation.
Various ways of compensating the response of a scintillation detector have been tried including providing the downhole detector with an oscillator for generating standard pulse heights, a light emitting diode for producing standard light flashes to be detected by the photomultiplier tube, a standard radioactive source and crystal to generate a known peak of pulses, or picking out a naturally existing peak of pulses to be used as the standard to correct any drift which may occur in the detector.
In U.S. Pat. No. 4,053,767 assigned to the assignee of the present invention a small crystal with a radioactive source is embedded into the main crystal and is placed adjacent to a photomulitplier tube such that the photomultiplier tube views the light pulses generated by the radioactive source. These standard light pulses are transmitted to the surface along with the data pulses sensed by the main crystal for analysis at the surface.
Samarium sleeves have also been used around the outside of the logging sonde to compensate for salinity effects.
In the present invention a radioactive source is placed in the end of the main crystal removed from the photomultiplier tube and is arranged such that the main crystal itself generates responsive to the reference source. Thus the reference pulses may monitor any changes in the response of the scintillating crystal as well as those changes in the photomultiplier tubes or the electronics of the detector. The reference pulses are divided by two window discriminators which are located downhole. The limits of the window discriminators are set such that the pulses in the first window are equal to the pulses in the second window at standard conditions. The frequency of pulses in each window are compared one with another and are integrated such that if the reference peak shifts, the integrator will produce a correction factor which may be used to shift the entire spectrum being sensed until the frequency of pulses in each window are again equal. All of the stabilization circuits are contained downhole such that only corrected pulses are transmitted to the surface.
The reference pulses are in that portion of the spectrum removed from the information bearing data. This arrangement provides that not later correction is necessary to remove the reference pulses from the spectrum being logged. The reference pulses are in the lower end of the energy spectrum. A cadmium shield is placed around the scintillation crystal to block low energy radiation from entering the crystal and masking the reference pulses.
The present invention also contains salinity compensation comprising samarium sleeves located on the inside of the logging sonde and spectrum discriminators whose limits are set such that compensation is made for the effects of salt in the borehole or formation fluids.